The overall objective of the proposed research is to understand the pathogenesis of Marek's Disease (MD) and to determine the means by which various factors affect pathogenesis. Specific objectives include: (1) studies to identify the nature of avirulence in the case of certain strains of MD virus, (2) continued studies on the character of infection and host response, especially the immune response, and (3) studies on the interaction between oncongenic DNA and RNA virus infections in dually-infected birds. All of these studies will utilize specific-pathogen-free birds of various genetic susceptibilities to the virus infections under investigation. Special emphasis will be directed toward the influence of the thymus and thymic-dependent immunity in controlling MD virus infections.